为什么Spring的FactoryBean getObject返回null？

Question

@Bean
@Autowired
public ConversionService conversionService(Set<Converter<?, ?>> converters) {
    final ConversionServiceFactoryBean factory = new ConversionServiceFactoryBean();
    factory.setConverters(converters);
    return checkNotNull(
            factory.getObject(),
            "conversionService must not be null.");
}

checknotnull引发NullPointerException。转换器按预期注入。为什么工厂返回null？我怎么才能修好？

Answer 1

由于Sotirios Delimanolis的评论，我得出了以下解决方案：

@Bean
public ConversionServiceFactoryBean conversionService(Set<Converter<?, ?>> converters) {
    final ConversionServiceFactoryBean factory = new ConversionServiceFactoryBean();
    factory.setConverters(converters);
    return factory;
}

这实际上是以下配置的简写：

@Bean
public ConversionService conversionService(Set<Converter<?, ?>> converters) {
    final ConversionServiceFactoryBean factory = new ConversionServiceFactoryBean();
    factory.setConverters(converters);
    factory.afterPropertiesSet(); // necessary
    return factory.getObject();
}

在调用AfterPropertiesSet（解释）之前，工厂一直处于未完成状态。但是，如果返回ConversionServiceFactoryBean而不是ConversionService本身，则不需要调用它。由于工厂是InitializingBean和FactoryBean，如果需要ConversionService实例，Spring将在内部调用AfterPropertiesSet和GetObject。